A power converter is a power processing circuit that converts an input voltage or current source into a specified output voltage or current. A DC/DC full-bridge phase-shift power converter is a frequently employed switched-mode power converter that converts an input DC voltage to a specified output DC voltage. The DC/DC full-bridge phase-shift power converter generally includes switching circuitry coupled to an input source of electrical power. The switching circuitry includes two pairs of alternately conducting active switches. A primary winding of a transformer is coupled to the switching circuitry and a secondary winding of the transformer is coupled to a rectifier (e.g., rectifying diodes). The rectifier circuit is coupled through an output filter to a load.
DC/DC converters, in general, can experience significant energy losses and voltage oscillations in the rectifier diodes. This problem is amplified when the DC/DC converters are employed in higher power applications. In the past, dissipative snubbers, such as RC snubbers, RCD snubbers or saturable inductors, have been used to suppress the voltage ringing associated with the rectifier diodes. While the dissipative snubbers adequately dampen the voltage oscillations, the dissipative snubbers increase power losses associated with the converter and, in the case of the saturable inductors, the saturable cores generate heat and cause thermal concerns in the converter at high operation temperatures.
Additional losses in switched-mode converters result from switching losses associated with the primary switches in the converter. To minimize the switching losses, transitioning the switches at minimal voltage or current is desirable. Zero-voltage switching can be achieved by employing phase shift control. Zero-current turn-off, however, is desirable in higher power applications since lower-cost and lower-conduction-drop isolated-gate bipolar transistors (IGBTs) may then be employed.
Recently, several topologies have been proposed to reduce the transformer primary current to zero before the resonant leg switches are turned off, aimed at reducing the turn-off losses of the switches. The proposed topologies generally employ a capacitor or an extra winding of the transformer or output choke to provide a discharging voltage to the transformer leakage inductance during a freewheeling mode thereof, thereby forcing the primary current to substantially zero. The proposed topologies, however, do not simultaneously resolve the rectifier reverse recovery condition. See, for instance, Zero Voltage and Zero Current Switch Full-Bridge PWM Converter for High Power Applications, by J. G. Cho, et al., pp. 102-108, PSEC 1994, which is incorporated herein by reference. Alternatively, an active snubber with two auxiliary switches has been proposed in A 48V, 1.5 kW, Front-End Zero-Voltage-Switched PWM Converter with Lossless Active Snubbers for Output Rectifiers, by D. B. Dalal, et al., pp. 722-728, APEC 1993, which is incorporated herein by reference. While Dalal proposes a circuit to manage and recover the energy associated with the reverse recovery current of the rectifier, it fails to even address the turn-off losses associated with the switching circuitry.
Accordingly, what is needed in the art is a snubber circuit that achieves a substantially zero current turn-off for the resonant leg switches of the converter and further reduces the reverse recovery current of the rectifier over a wide range of power applications.